In the area of portable devices that can efficiently and reliably identify chemical and biological agents, on-chip spectrometers provide an advantageous solution. A compact on-chip spectrometer can be realized in different configurations such as resonant wavelength filters like micro-ring resonators or Mach-Zender interferometers and dispersive components like echelle gratings, etched diffraction gratings, and arrayed-waveguide gratings (AWGs). AWG fabrication and device design have been steadily improving as AWGs are used in wavelength-division multiplexing for optical communication applications as well as on-chip spectrometer sensor applications. AWG designs with different materials and working wavelengths have been made for covering a wide spectral range for spectroscopy measurement.
However, one of the challenges in spectroscopic technologies is that the absorption spectrum of a real chemical system usually contains multiple wide and closely located bands of various compounds. Therefore, the total spectral curve is rather complex and broad, and some chemical components may only be partially and weakly shown in the spectra where the spectral resolution is insufficient for achieving a better spectral resolution (e.g., ≤2 nm).
Derivative spectroscopy measures the low intensity bands overlapped by bands of higher intensity in the absorption spectra by utilizing the derivatives of the absorption spectrum data for qualitative and quantification analysis. In the last several decades, derivative spectroscopy has had increased practical applications because the fast development of microcomputers has enabled the easy and fast generation of high sensitivity, high resolution and low noise high-order derivative spectra.
According to the Beer-Lambert law, the light absorbed by a layer of substance is described byI=I0×10−εCd  (1)where I0 and I are input and transmitted optical intensities, respectively, ε is a wavelength dependent coefficient, C is a concentration of a substance and d is a thickness of the substance layer. Absorbance is defined asA=log10(I0/I)  (2)and the absorbance as a function of wavelength λ, denoted asA=ƒ(λ),  (3)can be approximated by various types of functions, such as Gaussian equation and Lorentzian equation. However, as said before, a challenge in spectroscopic technologies is that the absorption spectrum of a real chemical system usually contains multiple wide and closely located bands of various compounds and each form of the absorption band may vary from known functions. Therefore, the total spectral curve is rather complex, and some components may be only partially or weakly displayed. One such spectral curve 100 is depicted in FIG. 1A where two closely located absorption bands 102, 104 cannot be resolved in the total absorption curve 100.
Derivative spectroscopy provides a powerful tool to solve such problems mentioned. Firstly, considering a single absorption band with a Gaussian curve shape 200, the Nth order derivative spectrum has N+1 bands, as shown in FIG. 2 (where the zeroth-derivative spectrum 202, i.e. the original Gaussian spectrum 200, has one band 204, the first derivative spectrum 210 has two bands 212, 214, the second derivative spectrum 220 has three bands 222, 224, 226, the third derivative spectrum 230 has four bands 232, 234, 236, 238, and the fourth derivative spectrum 240 has five bands 242, 244, 246, 248, 250). The added complexity in the derivative spectrum can provide much useful information for the qualitative analysis of the material. Secondly, the derivative spectrum can resolve closely placed absorption bands which cannot be resolved in the absorbance mode. In the spectral curve 100 of FIG. 1A, the two components of the absorbance 102, 104 cannot be resolved from the absorption spectrum. However, from the fourth-order derivative spectrum 150 shown in FIG. 1B, the two absorption bands 152, 154 can be clearly visible. Thirdly, the derivation of the absorption spectrum can be used to discriminate absorbance bands with different bandwidths but same amplitude. The amplitude of an Nth order Gaussian band derivative spectrum is inversely proportional to the bandwidth of the original spectrum to the Nth degree. Therefore, the Nth-order derivative of a narrower spectrum has a larger amplitude than that of a broader spectrum.
Derivative spectra are mainly generated by optical or electrical methods. For optical methods, the frequency of the incident light is modulated with a narrow frequency (o by electromechanical methods, such as oscillating or rotating mirrors, or light source modulation to obtain first order and second order derivative spectrum. For dual wavelength methods, two spectrometers with a fixed center wavelength difference scan simultaneously to obtain a first-order derivative spectrum. Electrical methods use differential circuits consisting of analog resistance and capacitance devices to process a measured signal from a spectrometer readout to obtain the derivative data. However, both methods require high cost equipment and complex optics design, and such systems are typically very bulky. In addition, to estimate a derivative spectrum from an original absorption spectrum, a resolution of 2 nm or less is required for the original absorption spectrum. Many of existing spectrometers providing absorption spectra do not achieve such high resolution.
It is advantageous to have a spectrometer that provides an absorption spectrum having a resolution of 2 nm or less for further spectrum derivate estimation and that can be miniaturized in size, preferably implementable as an on-chip spectrometer. Based on this spectrometer and the high-resolution absorption spectrum that is obtained, a spectroscopy system for generating one or more derivative spectra is readily realizable in a small size. There is a need in the art to have such spectroscopy system.